Which modifier is defined as referring to an outside laboratory?

Modifier 90 is used to indicate that a service or test was performed by a reference (or outside) laboratory rather than the laboratory associated with the billing provider. This modifier is especially relevant when laboratory tests are sent to an outside facility for processing, and it helps distinguish the location where the services took place. By utilizing this modifier, the billing accurately reflects that the laboratory services were conducted externally, which is essential for proper reimbursement and compliance within medical billing.

In contrast, Modifier 91 is used for repeat clinical diagnostic laboratory tests when the same test is performed on the same day for different reasons, while Modifier 92 has a specific application for certain tests that are modified due to the necessity of a second specimen or unusual circumstances. Modifier QW, on the other hand, is used for laboratory tests that are waived under the Clinical Laboratory Improvement Amendments (CLIA), indicating that the test can be performed with minimal technical skills. Each of these modifiers serves a distinct purpose, but in the context of referring to outside laboratories, Modifier 90 is the appropriate choice.